Semiconductor light emitting diode (LED) devices have been made since the early 1960s and currently are manufactured for usage in a wide range of consumer and commercial applications. The layers comprising the LEDs are based on crystalline semiconductor materials that require ultra-high vacuum techniques for their growth, such as, molecular organic chemical vapor deposition. In addition, the layers typically need to be grown on nearly lattice-matched substrates in order to form defect-free layers. These crystalline-based inorganic LEDs have the advantages of high brightness (due to layers with high conductivities), long lifetimes, good environmental stability, and good external quantum efficiencies. The usage of crystalline semiconductor layers that results in all of these advantages, also leads to a number of disadvantages. The dominant ones are high manufacturing costs, difficulty in combining multi-color output from the same chip, the need for high cost and rigid substrates.
In the mid 1980s, organic light emitting diodes (OLED) were invented (Tang et al, Appl. Phys. Lett. 51, 913 (1987)) based on the usage of small molecular weight molecules. In the early 1990's, polymeric LEDs were invented (Burroughes et al., Nature 347, 539 (1990)). In the ensuing 15 years organic based LED displays have been brought out into the marketplace and there has been great improvements in device lifetime, efficiency, and brightness. For example, devices containing phosphorescent emitters have external quantum efficiencies as high as 19%; whereas, device lifetimes are routinely reported at many tens of thousands of hours. In comparison to crystalline-based inorganic LEDs, OLEDs have much reduced brightness (mainly due to small carrier mobilities), shorter lifetimes, and require expensive encapsulation for device operation. On the other hand, OLEDs enjoy the benefits of potential lower manufacturing cost, the ability to emit multi-colors from the same device, and the promise of flexible displays if the encapsulation issue can be resolved.
To improve the performance of OLEDs, in the later 1990's OLED devices containing mixed emitters of organics and quantum dots were introduced (Matoussi et al., J. Appl. Phys. 83, 7965 (1998)). The virtue of adding quantum dots to the emitter layers is that the color gamut of the device could be enhanced; red, green, and blue emission could be obtained by simply varying the quantum dot particle size; and the manufacturing cost could be reduced. Because of problems, such as, aggregation of the quantum dots in the emitter layer, the efficiency of these devices was rather low in comparison with typical OLED devices. The efficiency was even poorer when a neat film of quantum dots was used as the emitter layer (Hikmet et al., J. Appl. Phys. 93, 3509 (2003)). The poor efficiency was attributed to the insulating nature of the quantum dot layer. Later the efficiency was boosted (to ˜1.5 cd/A) upon depositing a monolayer film of quantum dots between organic hole and electron transport layers (Coe et al., Nature 420, 800 (2002)). It was stated that luminescence from the quantum dots occurred mainly as a result of Forster energy transfer from excitons on the organic molecules (electron-hole recombination occurs on the organic molecules). Regardless of any future improvements in efficiency, these hybrid devices still suffer from all of the drawbacks associated with pure OLED devices.
Recently, a mainly all-inorganic LED was constructed (Mueller et al., Nano Letters 5, 1039 (2005)) by sandwiching a monolayer thick core/shell CdSe/ZnS quantum dot layer between vacuum deposited n- and p-GaN layers. The resulting device had a poor external quantum efficiency of 0.001 to 0.01%. Part of that problem could be associated with the organic ligands of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) which were reported to be present post growth. These organic ligands are insulators and would result in poor electron and hole injection onto the quantum dots. In addition, the remainder of the structure is costly to manufacture, due to the usage of electron and hole semiconducting layers grown by high vacuum techniques, and the usage of sapphire substrates.
Accordingly, it would be highly beneficial to construct an all inorganic LED based on quantum dot emitters which was formed by low cost deposition techniques and whose individual layers showed good conductivity performance. The resulting LED would combine many of the desired attributes of crystalline LEDs with organic LEDs.